1. Field of the Invention
The invention relates to projectors of images in which a spatial modulation of light is carried out by means of a liquid crystal display screen.
More particularly, it relates to the projectors of images using the two directions orthogonal and complementary directions of polarization of light.
2. Description of the Prior Art
In image projectors, the image projected on a screen results from a spatial modulation of light. The light produced by a source is guided up to a light modulation unit grouping together with the means needed to achieve the spatial modulation of light. It is common practice to use a liquid crystal display (LCD) matrix screen as a spatial modulator of light. This LCD matrix screen comprises a matrix network of elementary liquid crystal cells controlled by means of a video signal: each cell represents an elementary image dot.
The simplest way of forming an image by means of an LCD matrix screen is to illuminate it by means of a linearly polarized light. To do this, it is common practice firstly to interpose a polarizer between the light source and the LCD matrix screen in order to select a particular direction of polarization for the illumination of the LCD matrix screen and then, secondly, after the LCD matrix screen, to place a second polarizer (also called a "polarization analyzer") that converts the angular modulation of polarization produced by the LCD matrix screen into a modulation of intensity. The main drawback of this method is that, by the selection of a particular direction of polarization, for the illumination of the LCD matrix screen, more than half of the light energy is lost.
A known way to improve the efficiency of illumination of the LCD matrix screen is to separate the light spatially into two polarized beams having complementary directions of polarization in order to use all the light produced by the source. Several methods may be used to this effect:
a) In a first method (see European patent application No. 0.372.905), each polarized beam illuminates a different LCD matrix screen, and the images formed separately by the two LCD matrix screens are superimposed. The major drawback of this method is that it calls for the use of two LCD screens, the result of which is that the cost of the image projector is very high;
b) In another method, the direction of polarization of one of the two polarized beams is made to rotate by 90.degree. so that the direction of polarization is the same for both polarized beams. Then the two beams are directed so as to illuminate the LCD matrix screen. A structure such as this has been shown and described in the report (page 90) of the "EURODISPLAY" conference organized by S.I.D. in Amsterdam in 1990.
The structure described in the above-mentioned document has at least one major drawback which lies in the fact that it either leads to a relatively substantial bulkiness of the projector (the distance between the polarization separating element and the LCD matrix screen) needed to prevent the two beams from being propagated towards the LCD matrix screen in forming a relatively large angle between them, or makes it obligatory to use a wide-aperture projection objective. Another drawback of this structure is that it cannot be used to being the two polarized beams with symmetrical optical paths so that it becomes difficult to achieve perfect superimposition of the illumination spots formed by these two beams on the same LCD matrix screen, i.e. to superimpose the sections of these two beams.